There are a number of different shapes of auxiliary vane that have been developed and used in existing impellers.
In one example, shown in U.S. Pat. No. 4,664,592, and the contents of which are incorporated here by reference, a number of radial auxiliary vanes are used. Those auxiliary vanes are located on the face of the front or back shroud, with an annular projection around the outer ends of the auxiliary vanes, and with a channel extending through the annular projection between adjacent auxiliary vanes.
A problem with the auxiliary vanes, with or without annular projections at the periphery, is that tip vortices form (similar to wingtip vortices) which, when particles are entrained, can cause severe localised gouging wear of the periphery of the impeller and the adjacent side liners.
As the parts wear, the vortex which forms behind each projecting vane gets larger and stronger causing an ever increasing wear rate in the adjacent side liner.
Water pumps are known which include auxiliary vanes at a smaller diameter than the shroud and main vane diameter (which are usually identical). The reason this is done is not to reduce wear, but to reduce the axial hydraulic thrust acting on the impeller. The auxiliary vane diameter is sized to balance the hydraulic axial thrust.